The present invention relates generally to downhole video systems. More particularly, the present invention relates to a downhole video system using standard electrical transmission lines to transmit video. Most particularly, the present invention relates to a downhole video system and method for using standard electrical transmission lines to transmit video and other downhole information to the surface for an improved video depiction of conditions downhole.
Modern society depends upon the inexpensive and continued production of hydrocarbons. In view of a limited world hydrocarbon supply, keeping energy costs low requires continual improvement in well drilling technology. This quest for improved geological formation evaluation and hydrocarbon recovery requires a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole commonly is referred to as "logging". Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled and can be performed by several methods. In conventional oil well wireline logging, a probe or "sonde" is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, electrical sensors, and video cameras. The sonde typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or "wireline". The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
During drilling and production, a variety of conditions downhole may impede or preclude the retrieval of hydrocarbons from a well bore. FIG. 1 illustrates a hypothetical well bore 100 and five different levels 110, 120, 130, 140, 150 of the well bore. A different condition exists at each of these levels. Gas leaks into the well bore at level A 110, nothing comes into the well bore at level B 120, oil leaks into the well bore at level C 130, water leaks into the well bore at level D 140, and an object 155 occupies the bottom level E 150 of the well bore 100. Object 155 may, for example, be a piece of equipment that has been mistakenly dropped down the well bore or has been broken.
As can be appreciated by one of ordinary skill in the well drilling arts, a particular hydrocarbon stream, such as oil, is normally sought from a particular well bore. As such, water leakage at level D 140 and gas leakage at level A 110 are not desirable and should be eliminated or minimized, if possible. If the exact depth and character of a gas or water leak can be found, known corrective measures can stop the leaks, and so it is very important to learn the depth and nature of a leak. For obstructions and lost items 155 in the well bore, known "fishing" tools and techniques can usually remove object 155 from the well bore if the object can be seen. Were object 155 left in the well bore, drilling and downhole operations would be complicated and abandonment of the well may be the only option. Because of the extremely high cost of drilling a well bore 100, it is highly preferable to remove object 155 from the well bore.
Downhole video systems have been found useful in locating and identifying the problems depicted in FIG. 1, in addition to others. For example, the video camera system can detect turbulence created by a leak and may identify different fluids leaking into the well bore. Particulate matter flowing out through a hole can be detected. Obstructions in the well bore can be seen. Formation fractures and their orientations may be detected along with damaged, parted, or collapsed tubings and casings. Corrosion surveys can also be performed. Other causes for loss of production, such as sand bridges or malfunctioning flow controls such as valves, may be identified by the downhole video.
FIG. 2 shows one such downhole video system including an instrument probe. Shown are a well logging system 200 including a borehole or well bore 210 and well instrument probe 220 hanging by a support cable 230. Support cable 230 attaches to rotatable winch 235, surface controller 240 inside enclosure 245, and transportable platform 248. Support cable 230 must be capable of extending through pressure gland 250, lubricator risers 252, and main valve 255, all part of well head 260.
FIG. 3 illustrates a well instrument probe 220 and attached support cable 230 in a well bore 210. Also shown are cable head 240, camera head 250, light head 260, and legs 270 attaching lighthead 260 to camera head 250. The instrument probe 220 contains the remote video camera and other electrical equipment, and connects to the surface by an electrical instrument cable 230, thereby permitting transmission of electrical power to the video camera and communication of data from the video camera to the surface equipment.
Borehole 210 often is about 21.5 cm (8.5 in) in diameter, but many wells are relatively small in diameter, on the order of 4.5 cm (1.75 in). Consequently, the instrument probe and its cable designated for use in such a well are limited in their respective diameters. This can lead to practical problems when a high pressure well is involved. The well shown in FIG. 2 is capped to prevent the uncontrolled escape of high pressure well fluids. In order to insert a downhole video instrument into such a well, the video instrument must be forced into the well through the cap. As is well known in the art, small instruments are easier to insert into a high pressure environment because they present less surface area against which the high pressure well fluids can act. Thus, small differences in the diameters of downhole instrument cables can have a tremendous impact on the ease and expense of inserting the cable and an attached instrument into the well. However, small diameter transmission lines typically have severe bandwidth limitations. The prior art attempts to obtain adequate bandwidth between the downhole camera and a surface video monitor by employing co-axial cable or fiber optic cable. However, each of these solutions comes with severe drawbacks.
One drawback of coaxial cable for video transmission is the necessity of a progressively larger coaxial cable for longer well bores. Because the minimization of cable size is highly preferable, thick coaxial cable is not an ideal solution for downhole video transmission. And while fiber optic transmission lines have an adequately small diameter, they are very expensive and have a tendency to break under the severe stresses downhole. Ideally, neither coaxial nor fiber optic transmission lines would be necessary. Instead, the standard electrical transmission lines could be used to obtain satisfactory video at a surface location.
FIG. 4 illustrates a standard electrical transmission and support line used for and connected to a well instrument probe 220. Such a standard electrical transmission line is conventionally about 0.55 centimeters wide. Transmission line 400 includes a copper conductor 410 at the center of insulation 420 and first and second armored layers 430, 440 of strength member strands wound helicaly around the outer insular jacket in opposite directions. Prior art electrical transmission lines such as that shown have adequate diameter profiles, but lack the bandwidth required for video transmission. Presently, the data transmission rate of these lines is about 34 kilobytes per second, with envisioned rates of 200 kilobytes per second being achieved in the foreseeable future. Nonetheless, this is extremely slow in view of the fact that a minute of downhole video may occupy 50 megabytes of memory.
FIG. 5 illustrates a prior art video transmission system. The surface of the earth 550 divides the system into both subterranean components 500 and above-surface components 560. Included below-ground are downhole video camera 510 and analog cable transmitter circuits 520. A transmission line 525 carries data from analog cable transmitter circuits 520 to analog cable receiver circuits 530. TV monitor 540 attaches to analog cable receiver circuits 530 and displays the video received from downhole. Analog devices are used because heretofore analog transmission was the most effective way of transmitting video uphole.
Therefore, a downhole video system is needed that can use the standard electrical transmission line for video communication with surface components, without the need for coaxial or fiber optic transmission lines. Ideally, this invention would provide an easily interpretable indication of the conditions downhole. Such an invention preferably could be implemented with only minimal additional equipment downhole.